Uganda planted native trees across thousands of hectares of degraded forest; 20 years later, canopy cover and diversity reached primary forest levels

A forest does not recover all at once. In Uganda's Kibale National Park, thousands of hectares of land that had been cleared for agriculture are now showing measurable signs of recovery after decades of active tree planting. A study published in Ecology and Evolution titled Chronosequence Resampling Elucidates Tree Community and Forest Structure Recovery Patterns in Restored Tropical Rainforest examined how forests changed across nearly three decades of restoration. Led by Eveliina Korkiatupa of the University of Eastern Finland, the research compared 45 actively restored forest sites ranging from four to 26 years old with 10 primary forest reference sites. The researchers used vegetation surveys conducted in 2013 and repeated in 2021 to track changes in tree diversity, forest structure and community composition. They found that after about 20 years of planting, canopy cover and Simpson's diversity had reached levels similar to those recorded in the primary forest reference sites. The recovery was far from complete, however, with several important measures of forest structure and species composition still substantially different from primary forest.The restoration effort began after decades of illegal clearingThe restoration area lies in the southern part of Kibale National Park, where approximately 10,000 to 15,000 hectares lost forest cover between the 1970s and 1990s because of illegal land clearance for agriculture. In 1994, the Uganda Wildlife Authority and Face the Future launched a reforestation project, with active restoration beginning in 1995. Restoration has since involved planting trees, removing competing grass around seedlings, preventing fires and controlling invasive species including Lantana camara. By 2023, more than 4,400 hectares had been planted with local tree species. The planting programme used a mixture of species representing different stages of forest succession. Pioneer species included Bridelia micrantha, Spathodea campanulata and Shirakiopsis elliptica, while intermediate-successional species included Albizia gummifera, Croton macrostachys, Cordia africana, Warburgia ugandensis, Mimusops bagshawei and Prunus africana. Climax species included Gambeya albida and Uvariopsis congensis. The study sites examined by Korkiatupa and her colleagues were located within these actively restored areas.Canopy cover recovered before the forest fully caught upThe clearest signs of structural recovery appeared in the canopy. The researchers found that canopy cover in restored forests reached levels similar to those of the primary forest reference sites after roughly two decades of restoration. A closing canopy can alter conditions on the forest floor by reducing the amount of light reaching competing grasses, potentially creating conditions that allow additional tree species to establish and survive. Simpson's diversity, an index that incorporates both species richness and the relative abundance of species, also reached values comparable to the primary forest after 20 years of planting. Species accumulation analyses similarly showed that the oldest restored forests had reached the primary forest's level of tree taxa richness in 2021. But this did not mean that the restored forests had become equivalent to primary forest. Overall, restored sites still had lower tree taxa richness, basal area and stem density in the statistical comparisons, while tree height also remained significantly lower. The researchers therefore described the recovery as partial, with different components of the forest developing at different speeds.Big trees are still largely missing from the restored areasOne of the biggest gaps was the number and size of large trees. In the study, trees with a diameter at breast height of at least 20 centimetres were classified as large trees, and they remained less abundant in restored forests than in primary forest even after 20 to 26 years of planting. The researchers noted that large and old trees account for much of a forest's biomass and play important roles in carbon storage, forest architecture, habitat creation and the regulation of light, moisture and other conditions beneath the canopy. Measurements of planted trees showed that they generally reached the 20-centimetre diameter threshold after approximately 14 to 21 years. That does not mean a restored forest becomes fully mature once planted trees reach this size. The researchers cited previous work suggesting that recovery of large-tree structure and associated forest functions can require one to two centuries. They also noted that the understorey and smaller tree communities can change more quickly, while the composition of medium and large trees remains much more distinct from primary forest.The species mix is changing, but full community recovery will take much longerThe restored forests were also becoming more similar to primary forest in some biological characteristics, although the overall tree community remained clearly distinct. In the 2021 survey, the proportion of shade-tolerant and forest-interior species increased along the restoration age gradient, while forest-non-dependent species were more common in restored forests than in primary forest. The researchers recorded about 60 tree taxa in the restored forests, compared with more than 350 tree species previously recorded in Kibale's primary forest, leaving substantial scope for further colonisation by later-successional species. At least four late-successional taxa had reached the restored forests after 13 to 26 years of planting, although the researchers noted that some species could have originated from planted individuals rather than arriving through dispersal from primary forest. Community composition overall remained distinct from primary forest, and the authors cited research estimating that tropical forest communities can require at least 120 years to recover 90% of reference-site values. The Kibale study therefore presents restoration as a gradual process rather than a simple return to the original forest: canopy and some measures of diversity can recover within decades, while large-tree structure and the composition of the wider tree community continue changing for much longer.You use AI every day. Now get your AI Quotient. Take the AIQ test.